feat(Local env): Enhance local Django shell devex

[emyller]

make django-shell now uses shell_plus.

IPython is chosen over other options because it's the most actively maintained of alternatives.

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6650	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6650	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api-test:pr-6650	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6650	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6650	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6650	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.20%. Comparing base (196ebf3) to head (e3f74b7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6650   +/-   ##
=======================================
  Coverage   98.20%   98.20%           
=======================================
  Files        1298     1298           
  Lines       47108    47108           
=======================================
  Hits        46262    46262           
  Misses        846      846           

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[khvn26]

Yes!

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored		Feb 3, 2026 4:07pm
flagsmith-frontend-preview	Ignored		Feb 3, 2026 4:07pm
flagsmith-frontend-staging	Ignored		Feb 3, 2026 4:07pm